WP-11: contract and failing tests for package upgrades

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/crates/application/src/test_fakes.rs

@@ -368,3 +368,34 @@ impl InventoryRepository for MemInventory {
         Ok(self.0.lock().unwrap().clone())
     }
 }
+
+use domain::ports::{HostUpdater, LineSink};
+
+/// Records the requested packages and emits a few lines.
+#[derive(Default)]
+pub struct FakeUpdater {
+    pub calls: Mutex<Vec<Vec<String>>>,
+    pub fail: bool,
+}
+
+#[async_trait]
+impl HostUpdater for FakeUpdater {
+    async fn upgrade(&self, packages: &[String], out: &dyn LineSink) -> Result<(), DomainError> {
+        self.calls.lock().unwrap().push(packages.to_vec());
+        out.line("Reading package lists...");
+        out.line(&format!(
+            "Upgrading {} package(s)",
+            if packages.is_empty() {
+                "all".to_string()
+            } else {
+                packages.len().to_string()
+            }
+        ));
+        if self.fail {
+            return Err(DomainError::Unavailable(
+                "apt-get exited with status 100".into(),
+            ));
+        }
+        Ok(())
+    }
+}